Peristaltic pump(s) with only two motors. Have one motor drive a carriage with the other motor. That motor is driving a great that interfaces with whichever other gears to go to each of these pumps.
Now, I’d need a way to verify the gears were going to mesh, so probably multiple photo diodes and a laser? I could dive as many pumps add I want, but only one at a time.
More elegant solution could be a motor turn a wheel, on the wheel is a motor. The motor messages with the pump gear. This one would only be limited by the great ratios (ie how many gears I could get around a gear.
The first idea is better.
All for better gardening.